The invention relates to acoustic transducers in general and more particularly to transducers for the transmission and reception of acoustic signals in a continuous wave mode or consisting of bursts having an oscillating portion at a selected frequency, as can be used for the detection of fluid velocity in a pipe, or conduit, or for non-destructive testing of material. There are applications for which a transducer must be dry-coupled in order to avoid any bonding or liquid coupling between the transducer components. This is particularly the case where the active components of the transducer are directly or indirectly exposed to high temperature, high pressure and corrosive fluid as can be found in liquid sodium installations or for the measure of sanitary fluid flow.
Dry-coupled transducers have been used successfully in travel time difference flowmeters operating under such severe conditions in the environment.
An example of such prior art transducers can be found in U.S. Pat. No. 3,925,692 of Walter C. Leschek et al for a "Replaceable Element Ultrasonic Flowmeter Transducer" assigned to the same assignee as the assignee of the instant patent application. The prior art transducer is provided with an acoustically transmissive metal window sealed at one end of the transducer housing. A coil spring is mounted in the housing to force an electrode member against the piezoelectric member and the latter against the metal window serving as the opposite electrode. A closure member is secured at the other end of the transducer housing in order to apply compression forces to the coil spring while allowing electrical connection therethrough.
When the transducer device is used with a high temperature, high pressure or corrosive transmissive medium, it is desirable that the acoustic window be made of a high temperature resistant high strength or chemically resistant material.
Moreover, high temperature forbids the use of adhesive bonding in the transducer component assembly at the back of the acoustic window. Still, it is desirable to hold the piezoelectric element in place and to provide good acoustic coupling between the window and the piezoelectric element. Also, the transducer assembly must be readily assembled in situ, and replacement of the piezoelectric element must be possible whenever necessary. To that effect, it is desirable to be able to assemble the transducer components with all the care possible outside the housing, and to hold such assembly together as a unit in preassembled form before actual installation within the housing.
In addition, while meeting mechanical requirements under severe conditions, the transducer assembly must possess all desirable acoustic requirements. For flowmeter applications, a pair of transducers is typically used as acoustic transmitter and receiver, respectively. Thus, in the receiver the piezoelectric element is usually sandwiched between the acoustic window serving as one electrode and a backing member serving as the second electrode. The latter should be structured so that it does not couple with the ultrasonic wave and reflect as much energy as possible back to the piezoelectric element in a resonant fashion, thereby to maximize transducer efficiency. Similar requirements exist for the transmitter and receiver transducers.
The invention is applicable to vortex flowmeter technique, but not exclusively.
Vortex flowmeters could not be effectively with high temperature, high pressure, corrosive, and sanitary fluid flow measuring applications, because mounting of the transducer and the strut creating the vortices had to be made through the pipe walls. As a result, seals were needed whereas sealing is best provided with an integral mounting of the strut, thus, without crevices inside the pipe. Crevices can create a problem in sanitary applications where a thorough internal cleaning is required and when the liquid sodium is used as fluid, for instance in nuclear applications, should the transducers or the strut be mounted through the pipe wall. Crevices could thus cause trouble.
Therefore, when transducers are mounted so as to penetrate into the pipe, they must be so designed as to withstand the internal environment. In the type of applications mentioned previously, this means that the transducers must be metal-enclosed and ruggedly constructed. Therefore, they will be relatively large and may seriously disturb the inner pipe surface contour and hence produce local turbulence which could mask the vortex effect. In general, it is desirable that the transducers be external to the pipe and that the installation of the strut be such as not to create crevices or reduce pressure capability. Bonding transducers to the outside of the pipe is a way to provide good acoustic coupling but the bonding agents often limit high temperature rating. Moreover, bonded transducers are not easily replaceable.
For a vortex flowmeter, it is also desirable to minimize acoustic coupling into the transducer parts located behind the transducer element. Standing wave conditions in certain parts could cause variations in the frequency response of the transducers, if appreciable energy happens to be coupled into them. As with standing waves in the fluid between transducers, the effect is to create narrow response peaks closely spaced in frequency within the main response band of the transducers. This can make tuning difficult and could cause sharp signal level changes for a small drift in the transmitting frequency.